Environmental Geochemistry
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Environmental Geochemistry

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What is geochemistry?. The study of-chemical composition of the Earth and other planets-chemical processes and reactions that govern the composition of rocks and soils-the cycles of matter and energy that transport the Earth\'s chemical components in time and space-and their interaction with the
Environmental Geochemistry

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1. Environmental Geochemistry January 26, 2007

2. What is geochemistry? The study of -chemical composition of the Earth and other planets -chemical processes and reactions that govern the composition of rocks and soils -the cycles of matter and energy that transport the Earth's chemical components in time and space -and their interaction with the hydrosphere and the atmosphere. wikipediawikipedia

3. Outline of Topics Formation of the elements Composition of Earth Aqueous Solutions Chemical Equilibrium Acid-Base Equilibria Redox Biogeochemistry Stable Isotopes (with comments on weathering, sorption, pollution?)

4. Formation of the Elements Fig 1 -Universe produced by Big Bang 13.7 billion years ago - based on measurements of the cosmic microwave background and red shift -protons and neutrons combined to form light nuclei (H and He) -fusion of H to form He nuclei in stars -once H in star used up, He nuclei fusion to form C, O -C fusion to form Si (also Na, Mg, Ne)? -Si burning to form elements near Fe -can?t get elements heavier than Fe (mass 56) with direct fusion - formation proceeds by supernovae and neutron capture Fig 2 -H and He are most abundant in solar system -abundances of Li, Be, and B anomalously low -abundances of elements whose atomic numbers greater than 6 (C) decrease exponentially with increasing Z -Fe exceptionally abundant, abundance of F lower than expected -Tc and Pm do not occur naturally in solar system (all isotopes unstable) -abundance of Pb somewhat greater than expected (stable end product of decay of U and Th) -U has lowest abundance of any element in solar system (except as noted above) -abundances of elements between Bi through Th low because these elements are unstable daughters of decay of naturally occurring isotopes of U and ThFig 1 -Universe produced by Big Bang 13.7 billion years ago - based on measurements of the cosmic microwave background and red shift -protons and neutrons combined to form light nuclei (H and He) -fusion of H to form He nuclei in stars -once H in star used up, He nuclei fusion to form C, O -C fusion to form Si (also Na, Mg, Ne)? -Si burning to form elements near Fe -can?t get elements heavier than Fe (mass 56) with direct fusion - formation proceeds by supernovae and neutron capture Fig 2 -H and He are most abundant in solar system -abundances of Li, Be, and B anomalously low -abundances of elements whose atomic numbers greater than 6 (C) decrease exponentially with increasing Z -Fe exceptionally abundant, abundance of F lower than expected -Tc and Pm do not occur naturally in solar system (all isotopes unstable) -abundance of Pb somewhat greater than expected (stable end product of decay of U and Th) -U has lowest abundance of any element in solar system (except as noted above) -abundances of elements between Bi through Th low because these elements are unstable daughters of decay of naturally occurring isotopes of U and Th

5. Composition of Earth Accretion Models for Earth Formation Homogenous Accretion (COLD): Accretion began after nebula was cool enough to have both metallic and silicate particles. Accumulation of planetesimals formed a well-mixed earth. Gravitational collapse released heat causing partial melting. Fe sank into the core (the ?iron catastrophe?; LIL?s rose to the crust Heterogenous Accretion (HOT): Proto-Earth accreted from dense Fe-rich particles that first condensed from solar nebula. After cooling of the nebula, Si-O rich particles formed which then accreted later to form the mantle and crust. Accretion Models for Earth Formation Homogenous Accretion (COLD): Accretion began after nebula was cool enough to have both metallic and silicate particles. Accumulation of planetesimals formed a well-mixed earth. Gravitational collapse released heat causing partial melting. Fe sank into the core (the ?iron catastrophe?; LIL?s rose to the crust Heterogenous Accretion (HOT): Proto-Earth accreted from dense Fe-rich particles that first condensed from solar nebula. After cooling of the nebula, Si-O rich particles formed which then accreted later to form the mantle and crust.

6. Chalcophile elements - high bonding affinity?usually in the form of covalent bonds?with sulfur, and are, accordingly, usually abundant in sulfides. - exhibit a bonding affinity with selenium, tellurium, arsenic, and antimony -When sulfur is abundant, chalcophile elements readily form sulfide minerals as they precipitate from the magma. This process partially explains the formation of extensive deposits of iron-nickel-copper sulfides. Lithophiles - high bonding affinity with oxygen. - affinity to form ionic bonds and are represented by silicates (silicon and oxygen) in the crust and mantle -Other lithophile elements include magnesium, aluminum, sodium, potassium, iron, and calcium. Siderophiles -exhibit a weak affinity to both oxygen and sulphur. -affinity for iron -high solubility in molten iron. -generally have a low reactivity -affinity to form metallic bonds. -most often found in their native state. -Not abundant in the core or mantle -most siderophiles are thought to be richest at Earth's core. -Platinum (Pt) group metals, including Ruthium (Ru), Rhodium (Rd), Palladium (Pd), Osmium (Os), and Iridium (Ir), show exhibit a strong siderophile tendency. Atmophiles are a related fourth class of elements characterized by their ability to form van der Waals bonds. Atmophiles are also highly volatile. Chalcophile elements - high bonding affinity?usually in the form of covalent bonds?with sulfur, and are, accordingly, usually abundant in sulfides. - exhibit a bonding affinity with selenium, tellurium, arsenic, and antimony -When sulfur is abundant, chalcophile elements readily form sulfide minerals as they precipitate from the magma. This process partially explains the formation of extensive deposits of iron-nickel-copper sulfides. Lithophiles - high bonding affinity with oxygen. - affinity to form ionic bonds and are represented by silicates (silicon and oxygen) in the crust and mantle -Other lithophile elements include magnesium, aluminum, sodium, potassium, iron, and calcium. Siderophiles -exhibit a weak affinity to both oxygen and sulphur. -affinity for iron -high solubility in molten iron. -generally have a low reactivity -affinity to form metallic bonds. -most often found in their native state. -Not abundant in the core or mantle -most siderophiles are thought to be richest at Earth's core. -Platinum (Pt) group metals, including Ruthium (Ru), Rhodium (Rd), Palladium (Pd), Osmium (Os), and Iridium (Ir), show exhibit a strong siderophile tendency. Atmophiles are a related fourth class of elements characterized by their ability to form van der Waals bonds. Atmophiles are also highly volatile.

7. Aqueous Solutions Compared with hydrides of other elements in same column on periodic table water is odd. It has a larger liquid temperature range and higher melting and boiling point than expected. Dipole moment, H bonding. Because of H bonding has a much higher boiling point than liquids of a similar mass (CH4). Max density at 4 degrees C. - influences stratification and mixing of lakes.Compared with hydrides of other elements in same column on periodic table water is odd. It has a larger liquid temperature range and higher melting and boiling point than expected. Dipole moment, H bonding. Because of H bonding has a much higher boiling point than liquids of a similar mass (CH4). Max density at 4 degrees C. - influences stratification and mixing of lakes.

8. Like dissolves like Water able to dissolve huge quantities of salts, therefore and important factor in transporting substances in nature More than 300g NaCl can be dissolved in 1kg of waterLike dissolves like Water able to dissolve huge quantities of salts, therefore and important factor in transporting substances in nature More than 300g NaCl can be dissolved in 1kg of water

9. TDS - total dissolved solids - combined content of all inorganic and organic substances contained in a liquid which are present in a molecular, ionized or micro-granular (colloidal sol) suspended form, go through 2um filter In solutions with high ionic strength you start to see deviations - Have to use activities (effective concentration) instead of concentrations. Non polar solutes lead to less deviation from ideal behavior than ions (for ions the electrostatic interactions are greater). Activity of water in dilute aqueous solutions is 1. Activity in sea water 0.98.TDS - total dissolved solids - combined content of all inorganic and organic substances contained in a liquid which are present in a molecular, ionized or micro-granular (colloidal sol) suspended form, go through 2um filter In solutions with high ionic strength you start to see deviations - Have to use activities (effective concentration) instead of concentrations. Non polar solutes lead to less deviation from ideal behavior than ions (for ions the electrostatic interactions are greater). Activity of water in dilute aqueous solutions is 1. Activity in sea water 0.98.

10. Chemical Equilibrium

11. Reaction Rates/Equilibrium

12. Acid-Base Equilibria pH - measure of proton activity HA --> A- + H+ pH = pKa + Log [A-]/[HA] pH = -Log[H+]pH - measure of proton activity HA --> A- + H+ pH = pKa + Log [A-]/[HA] pH = -Log[H+]

13. Acid-Base Oxidation often produces acidity, reduction consumes acidity. Photosynthesis and respiration - also acid base reactions - photosynthesis consumes acidity, respiration produces acidity. Decrease in pH also accelerates weathering processes - some of the most common elements in rocks (Al3+ and Fe3+) become much more soluble at lower pH. In forest ecosystems etc lower pH accelerates leaching of base cations from soil which can negatively impact forest growth. H+ will exchange for other cations - Na+, K+, etc. pH can also influence sorption properties of species - above isoelectric point for minerals there is net negative surface charge, below there is net positive surface charge. H2PO4- sorbs less as pH increases because of electrostatic interactions with surface.Oxidation often produces acidity, reduction consumes acidity. Photosynthesis and respiration - also acid base reactions - photosynthesis consumes acidity, respiration produces acidity. Decrease in pH also accelerates weathering processes - some of the most common elements in rocks (Al3+ and Fe3+) become much more soluble at lower pH. In forest ecosystems etc lower pH accelerates leaching of base cations from soil which can negatively impact forest growth. H+ will exchange for other cations - Na+, K+, etc. pH can also influence sorption properties of species - above isoelectric point for minerals there is net negative surface charge, below there is net positive surface charge. H2PO4- sorbs less as pH increases because of electrostatic interactions with surface.

14. Acid-Base People often only consider carbonate alkalinity but total alkalinity also includes NH3, [B(OH)4]-, and [HPO4]2- Most natural waters have positive values for ANC. Waters impacted by acid mine drainage may have negative ANC. if a process consumes Ca2+ without affecting the concentration of any other base cation or acid anion, then charge balance must be maintained through release of H+ or through consumption of OH- People often only consider carbonate alkalinity but total alkalinity also includes NH3, [B(OH)4]-, and [HPO4]2- Most natural waters have positive values for ANC. Waters impacted by acid mine drainage may have negative ANC. if a process consumes Ca2+ without affecting the concentration of any other base cation or acid anion, then charge balance must be maintained through release of H+ or through consumption of OH-

15. Redox

16. Redox Rxns occur as redox couples - don?t have free floating electrons Oxidation reaction examples Fossil fuel --> CO2 (C as 0 to C as +4) Fe --> rust (Fe as 0 to Fe as +3) Reduction rxn examples SO4 2- --> H2S (gas that smells like rotten eggs, S as +6 to S as -2)Rxns occur as redox couples - don?t have free floating electrons Oxidation reaction examples Fossil fuel --> CO2 (C as 0 to C as +4) Fe --> rust (Fe as 0 to Fe as +3) Reduction rxn examples SO4 2- --> H2S (gas that smells like rotten eggs, S as +6 to S as -2)

17. Why do we care about redox rxns? (relative strengths of oxidant or reductant depend on pH) 1- (related to solubility) As oxidation state influence sorption to Fe oxides. Presence or absence of Fe oxides influences sorption of Zn2+ and Pb2+. 2- As III more soluble than As V. Oxidized U more soluble than reduced U. Cr VI more soluble than Cr III. Under reducing conditions Hg forms organometallic complexes which are mobile (and toxic). Cr VI more toxic. As III more toxic. 4. NH3 (reduced N) easier to incorporate into bacterial biomass. Can take up NO3- but have to expend energy to convert it to reduced N.(relative strengths of oxidant or reductant depend on pH) 1- (related to solubility) As oxidation state influence sorption to Fe oxides. Presence or absence of Fe oxides influences sorption of Zn2+ and Pb2+. 2- As III more soluble than As V. Oxidized U more soluble than reduced U. Cr VI more soluble than Cr III. Under reducing conditions Hg forms organometallic complexes which are mobile (and toxic). Cr VI more toxic. As III more toxic. 4. NH3 (reduced N) easier to incorporate into bacterial biomass. Can take up NO3- but have to expend energy to convert it to reduced N.

18. Accumulation of O2 in the Atmosphere Fe(III) many orders of magnitude less soluble than Fe(II) Large iron deposits formed between 2.5 and 2 billion years ago, require very low but not zero free oxygen in the atmosphere to form Accumulation of oxygen in atmosphere requires burial of significant amount of organic carbon - stashed as coal, oil, tar, shales, methane Detailed accumulation history of oxygen not known, these pictures represent a possibility Evidence for oxygen accumulation also in S isotopes (mass independent fractionation)Fe(III) many orders of magnitude less soluble than Fe(II) Large iron deposits formed between 2.5 and 2 billion years ago, require very low but not zero free oxygen in the atmosphere to form Accumulation of oxygen in atmosphere requires burial of significant amount of organic carbon - stashed as coal, oil, tar, shales, methane Detailed accumulation history of oxygen not known, these pictures represent a possibility Evidence for oxygen accumulation also in S isotopes (mass independent fractionation)

19. Biogeochemistry

22. 

23. What is an isotope? Isotope- line of equal Z. It has the same # protons (ie. they are the same element) but a diff. # of neutrons.

24. 4 types of isotopes, based on how they formed: Primordial (formed w/ the universe) Cosmogenic (made in the atmosphere) Anthropogenic (made in bombs, etc) Radiogenic (formed as a decay product) How did all this stuff get here?

25. Stable Isotopes Heavy molecules have stronger covalent bonds, less reactiveHeavy molecules have stronger covalent bonds, less reactive

26. Rayleigh fractionation: light isotopes evaporate more easily, and heavy isotopes rain out more quickly Stable Isotope Examples As you move inland precip is more depleted in heavy isotopes. Temp dependence of 18O and D in precipitated snow in polar regions used to interpret the isotope composition of ice cores recovered from ice sheets in Greenland and Antarctica and ice cores from high mountains. Taken together, these ice cores contain a record of climatic conditions during and subsequent to the Wisconsin glaciation, not only in the polar regins but also at midlatitudes in different places of the Earth.- many complications in interpreting these records though.As you move inland precip is more depleted in heavy isotopes. Temp dependence of 18O and D in precipitated snow in polar regions used to interpret the isotope composition of ice cores recovered from ice sheets in Greenland and Antarctica and ice cores from high mountains. Taken together, these ice cores contain a record of climatic conditions during and subsequent to the Wisconsin glaciation, not only in the polar regins but also at midlatitudes in different places of the Earth.- many complications in interpreting these records though.

27. d18Ocarbonate in forams depends on d18Oseawater as well as T, S d18Oseawater depends on how much glacial ice there is Glacial ice is isotopically light b/c of Rayleigh fract. More ice means higher d18Oseawater Stable Isotope Examples Also use O isotopes for thermometry of silicates and oxides - rock forming silicates and oxide minerals fractionate O isotopes when they crystallize from silicate melts or form by recrystallization of volcano-sed rocks during regional metamorphismAlso use O isotopes for thermometry of silicates and oxides - rock forming silicates and oxide minerals fractionate O isotopes when they crystallize from silicate melts or form by recrystallization of volcano-sed rocks during regional metamorphism

28. Stable Isotopes C in organic matter, fossil fuels, and hydrocarbon gases is depleted in 13C ==> photosynthesis used as an indicator of their biogenic origin and as a sign for the existence of life in Early Archean time (~ 3.8 billion years ago) N isotopic composition of groundwater strongly affected by isotope fractionation in soils plus agricultural activities (use of N-fertilizer and discharge of animal waste) Particulate matter in ocean enriched in 15N by oxidative degradation as particles sink through water column Used for mixing and sedimentation studies S isotopes fractionated during reduction of SO42- to S2- by bacteria didn?t become important until after ~2.35 Ga when photosynthetic S-oxidizing bacteria had increased sulfate concentration in the oceans sufficiently for anaerobic S-reducing bacteria to evolve (photosynthesis preceded S-reduction which was followed by O respiration) C isotope fractionation during photosynthesis - different types of plants have different levels of 13C depletion - C3 plants fractionate more than C4 plants Fractionation of N isotopes during fixation, nitrification, and denitrification Bacterial reduction of S enriches H2S in 32S by 50 per mil or more wrt sulfate In general - can be used to tell us about biological and earth processesC isotope fractionation during photosynthesis - different types of plants have different levels of 13C depletion - C3 plants fractionate more than C4 plants Fractionation of N isotopes during fixation, nitrification, and denitrification Bacterial reduction of S enriches H2S in 32S by 50 per mil or more wrt sulfate In general - can be used to tell us about biological and earth processes

29. Stable isotopes can also tell you about biology Organisms take up light isotopes preferentially So, when an organism has higher ?30Si, it means that it was feeding from a depleted nutrient pool Stable Isotope Examples

30. Stable Isotopes B isotopes can also give evidence of wastewaterB isotopes can also give evidence of wastewater

31. References http://mineral.gly.bris.ac.uk/Geochemistry/ http://mineral.gly.bris.ac.uk/envgeochem/ http://www.soest.hawaii.edu/krubin/gg425-sched.html http://geoweb.tamu.edu/courses/geol641/notes.html http://www.imwa.info/Geochemie/Chapters.HTML (WM White Geochemistry Ch9 - Stable Isotopes) Isotopes: Principles and Applications - Faure & Mensing How to Build a Habitable Planet - Wally Broecker


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